1. Field of the Invention
The present invention relates to flip chip assembly structure for a semiconductor chip in which each of precious metal bumps is employed as a material for electrical connection.
2. Description of the Related Art
As for the conventional flip chip assembly method employing Au bumps, 1) the assembly method wherein insulating resin is applied to an insulating board, the position of the insulating board is made align with the position of a semiconductor element, the load is applied to make the projection electrodes of the semiconductor element contact the conductor wiring of the insulating board and also then the insulating resin is cured to carry out the connection is described in JP-A-2-28946, and 2) the assembly method wherein the heating is carried out through an anisotropic conductive adhesive and also both of the pressure and the ultrasonic wave are applied to connect bumps which are formed on a chip and electrodes of a board to each other is described in JP-A-11-26922. In addition, 3) the assembly method wherein bumps which are formed on a chip and connection parts of a board are connected to each other by the heating and the application of the pressure while applying thereto the ultrasonic wave is described in JP-A-10-107078.
In the light of the foregoing, the present invention has been made in order to solve the above-mentioned problems associated with the prior art, and it is therefore an object of the present invention to provide, in assembly structure in which a chip is mounted to a printed circuit board in a face down manner, flip chip assembly structure in which the temperature cycle reliability and the electrical characteristics of the electrical connection parts are excellent.
In addition, It is another object of the present invention to provide, in a method of flip-chip mounting a semiconductor chip having a plurality of precious metal bumps formed thereon to a printed circuit board, a flip chip assembly method, having high productivity and high joining reliability, wherein the metal joining between precious metal bumps and internal connection terminals of a printed circuit board, and filling a space defined between the chip and the board with resin can be carried out in one process, electrical resistances of the connection parts can be reduced, and the under fill filling which is void free can be made using resin having a high filler containing rate.
In the assembly structure by the conventional assembly method 1), it is difficult due to the problem such as the heat resistance or the like of the under fill resin to increase the heating temperature so that the metallic joining between the bumps and the pads (in the present specification, the joining at the atom level is referred to as the metal joining) is not achieved. In addition, since resin is left on the interface, the contact resistance becomes considerably large and is in the range of several tens to one hundred and several tens mxcexa9. In the future, a semiconductor chip has a tendency in which the low voltage driving will advance more and more. In such a case, if the connection resistance is large, then it becomes difficult to operate normally the circuits on the chip. In addition, the loss due to the resistances in the connection parts is large, and hence it becomes a problem in terms of the power consumption. Also, if resin draws the moisture to expand under the environment of the high temperature and the high humidity, then the resistance is increased, and in the extreme case, the malconduction is caused. This becomes a problem in terms of the reliability as well.
In the assembly structure by the conventional assembly method 2), since the electrical conduction is ensured on the basis of the contact between the electrically conductive particles, there arises a problem that it is difficult to achieve the metal joining and hence the electrical resistance in each of the connection parts is large. In addition, when each of the bumps becomes minute, the number of electrically conductive particles which are caught fluctuates, and hence there is the possibility that the electrical characteristics of the connection parts become unstable.
In addition, in the assembly structure by the conventional assembly method 3), since while the metal joining is achieved, in this method, resin is not previously filled into the space defined between the chip and the board, there is adopted the process wherein after completion of the metal joining, liquid resin is led into that space by utilizing the capillary phenomenon to fill the under fill. In the assembly structure by this method, when the chip area becomes large or when the gap defined between the chip and the board becomes small, there arises a problem that the air layer is left due to the fact that the speed at which the resin flows thereinto differs depending on the places so that the void defects may be easy to be generated. In addition, in the extreme case, it may be difficult to fill resin in itself therein in some cases. In particular, this problem becomes more remarkable as the containing rate of inorganic fillers becomes higher which is mixed therewith in order to reduce the coefficient of thermal expansion.
By the way, by the void discussed herein is meant the void having the size (the diameter is about 30 xcexcm) in which the force which is caused by the volume expansion (cubical expansion) (1.2xc3x97103 times) when water is changed from liquid to gas and which serves to expand the space defined between the chip and the board exceeds {fraction (1/10)} (1 g) of the level at which one bump joining part is peeled off.
According to one aspect of the present invention, there is provided flip chip assembly structure including: a semiconductor chip having a circuit for processing electrical signals; electrodes which are provided on the semiconductor chip; bumps which are respectively formed on the electrodes; internal connection terminals through which the electrical signal is fetched from the associated ones of the electrodes via the associated ones of the bumps; and a printed circuit board on which the internal connection terminals are provided, wherein a semi-cured resin sheet which is softened by the heating is inserted into the space defined between the semiconductor chip and the printed circuit board, and the load is applied thereto, the heating is carried out and the ultrasonic wave vibration is applied so that the bumps and the internal connection terminals are metallically joined to each other.
It is desirable that each of the bumps is made of precious metal. In particular, Au is desirably employed therefor.
In addition, it is preferable that the melting point of a metal material of which each of the metallically joined connection parts is made is equal to or higher than 270xc2x0 C. The present invention may provide that the resin sheet which is inserted into the space defined between the semiconductor chip and the printed circuit board contains 50 vol % or more inorganic fillers.
In addition, the metallic electrodes of the semiconductor chip and the internal connection terminals of the printed circuit board are electrically connected to each other through the precious metal bumps by the metal joining; the melting point of a metal material of which each of the connection parts is made is equal to or higher than 275xc2x0 C.; the resin (under fill) containing 50 vol % or more inorganic fillers is led into the space defined between the chip and the board; resin is formed in such a way as to become void-free; and the fluctuation, of the containing rate of inorganic fillers contained in resin, which is dependent of the places is made equal to or lower than 10% or less (with respect to the definition of the fluctuation, the fluctuation of the filler containing rate is obtained by cutting out resin of 1 millimeters square from an arbitrary place within the chip surface, and the value which is obtained by dividing the difference between the maximum value and the minimum value thus obtained from that fluctuation by the mean containing rate is expressed in the form of the fluctuation rate).
In addition, according to the present invention, there is provided a method wherein projection bumps, made of precious metal, each of which has a projection type shape are respectively formed on electrodes of a semiconductor chip; precious metal is formed on the surfaces of internal connection terminals of a printed circuit board; a thermosetting resin sheet which has been semi-cured by mixing therewith fine inorganic fillers is attached to a predetermined position of the printed circuit board; the printed circuit board is set on a heat stage; the chip is mounted thereto in a face down manner with the bumps and the internal connection terminals aligned with each other; a heated joining tool for applying the ultrasonic wave and the load is pressed against the chip from the upper side by a predetermined force; and after the precious metal bumps have been buried in the resin sheet, which was softened by the heating, to come into contact with the connection pads, respectively, the precious metal bumps are crushed to be metallically joined to the pads, respectively, while pushing out a part of the resin sheet from the part between the chip and the board to the outside by applying thereto the ultrasonic wave vibration.
According to this method, since the bonding is carried out while pushing out a part of resin from the part between the chip and the board, the generation of the voids each having a size equal to or larger than the space defined between the chip and the board can be prevented with the probability of 100%. Further, if even when the filler containing rate is high, this rate falls within the range in which the sheet can be produced, the space defined between the chip and the board is filled with the fillers with the fillers uniformly dispersed in that space. Thus, the assembly in which the filling quality of organic resin is fixed becomes possible. In this method, since resin which is excellent in the thermal stability and the hygroscopic property can be used irrespective of the fluidity, there are offered the advantages that the high temperature stability of resin after completion of the curing processing by the baking can be enhanced, and also the problem of the corrosion or the like under the high temperature and high humidity environment due to the material generated through the thermal decomposition of resin, and the problem of the degradation of resin due to the hydrolysis can be reduced.
On the other hand, in the method employing that resin sheet, since resin lies between the bumps and the terminals, there is the possibility that a problem arises in the joining between the bumps and the pads. In actual, the metallic joining can not be achieved due to the remaining thin resin layer by the compression joining which is simply based on the heating and the application of the pressure. For this reason, there arise a problem that the contact resistance of the connection part is increased up to several tens to several hundreds mxcexa9, and a problem that the breaking of wire(s) is easy to occur due to the long term change, the expansion change resulting from the moisture drawing and the like of resin. Then, in the present invention, in order to achieve the metallic joining, there has been desired a method utilizing the ultrasonic wave vibration provided by a novel joining machine. As a result, resin can be exhausted from the composition interfaces between the bumps and the internal connection terminals to achieve the metallic joining. The details of the connection machine will be described later.
While in the above description, the specific case where each of the pad surfaces on the board side is made of the precious metal film has been described, even when each of the pad surfaces on the board side is made of a low-melting point film, by adopting the same assembly method, the under fill which has the high filter filling rate and the high molecular weight can be filled in the space defined between the chip and the board in the void free manner and also the connection part having a low resistance and high temperature reliability can be realized. The reason that the high temperature reliability of the joining parts can be enhanced is that in the ultrasonic wave flip chip joining process, low-melting metal or an eutectic alloy which has been melted can be exhausted from the composition interfaces to provide the structure in which the joining is carried out using only high-melting point metal or a high-melting point alloy.
As has been described in detail, according to the present invention, it is possible to provide flip chip assembly structure in which the electrical connection parts are excellent in the heat resistance and the electrical characteristics.